1. Field of the Invention
This invention relates to a process for analysing gas emitted during drilling of a borehole, especially hydrocarbon gas.
2. Background of the Art
The exploration and recovery of hydrocarbons from reservoirs is an expensive and complex process. Once a potential reservoir is identified an exploratory well may be drilled in order to determine the amount and type of hydrocarbons present. Once satisfied that there is sufficient hydrocarbons present to make it commercially worthwhile to extract, a well is also drilled which will eventually be used to recover the hydrocarbons.
In either case it is useful to know as much as possible about the nature of the reservoir, e.g. water content, porosity, and particularly the hydrocarbons present, e.g. their quantity, chain length etc. In the case of exploratory wells, this information may be used to determine whether to proceed with extracting hydrocarbons from the reservoir, and in the case of drilling a producing well, the information can be used to modify remaining options in the well design, and/or project future returns from the well. Information can also be useful for other types of wells, e.g. injection wells.
It is known to analyse the formation gases emitted during drilling, using a mass spectrometer, which elicits information as to the nature of the reservoir especially the hydrocarbons in the reservoir. Whereas conventional FID gas chromatography provides information about chain alkanes with 1 to 5 carbon atoms in each molecule (methane-pentane), mass spectrometry provides information about for example chain alkanes with 1 to 10 or more carbon atoms in each molecule (methane-decane), cyclic alkanes with 5 to 10 or more carbon atoms in each molecule, aromatic compounds with 6 to 10 or more carbon atoms in each molecule and a number of inorganic gases including carbon dioxide, nitrogen, hydrogen and helium.
When drilling, it is conventional to use a drilling fluid sometimes called ‘drilling mud’. The drilling fluid serves to cool a drill bit used to cut into the formation, lubricates the drill bit and also suspends and transfers the drill cuttings back to the surface.
The use of oil-based mud (OBM) (including synthetic oil-based mud (SOBM)) for drilling fluid is becoming increasingly popular because it helps to better lubricate the drill bit and mitigates formation damage. OBM including SOBM contains volatile organic compounds (VOCs) which mix with the gas from the formation and so affect the mass spectrometer results. Accordingly, determining the portion of the results which are due to the formation gas (and which are indicative the nature of the reservoir and especially the hydrocarbon content) and those that are due to the VOCs in the mud is challenging. The use of OBM or SOBM thus results in less accurate or simply less information on the nature of the reservoir and its hydrocarbon content.
The inventors of the present invention have noted that the precise VOC content of OBMs and SOBMs vary from product to product and even batch to batch, and indeed the proportions of the different ingredients in drilling mud is often varied in order to best suit the particular conditions of the borehole being drilled. Accordingly the chemistry of the mud and its fingerprint mass spectrum varies widely. As a further complication it is thought that the temperature variance in the borehole, which tends to increase the deeper it gets, causes a differing amount and type of VOCs to be released from the drilling mud. Thus interpretation of spectra produced from gases released from the borehole during drilling is very difficult if not impossible, when using oil based mud, because of the uncertain and variable composition of the gases emitted from the oil based mud.
An object of the present invention is to provide more information on the nature of the reservoir and especially the nature of the hydrocarbon content therein, based on mass spectrometer analysis of the gases recovered during drilling a borehole.